A hammer drill typically includes a tool holder in which a cutting tool, such as a drill bit, can be supported and driven by the hammer drill. The hammer drill can often drive the cutting tool in three different ways, each being referred to as a mode of operation. The cutting tool can be driven in a hammer only mode, a rotary only mode and a combined hammer and rotary mode.
A hammer drill will typically comprise an electric motor and a transmission mechanism by which the rotary output of the electric motor can either (a) rotationally drive the cutting tool to perform the rotary only mode or repetitively strike the end of a cutting tool to impart axial impacts onto the cutting tool to perform the hammer only mode or (b) rotationally drive and repetitively strike the cutting tool to perform the combined hammer and rotary mode. European Patent Application No. EP1674207 describes an example of such a hammer drill.
US Publication No. 2005/0173139 describes an impact driver with a tool holder in which a tool, such as a screw driver bit, can be supported and rotationally driven by the impact driver. The impact driver has a tangential impact mechanism which is activated when a large torque is experienced by the tool. The tangential impact mechanism imparts tangential (circumferential or rotational) impacts onto the tool until the torque applied to the tool drops below a predetermined value.
It is known to provide hammer drills with an additional tangential impact mechanism so that the hammer drill can impart rotational impacts onto a cutting tool in addition to axial impacts. U.S. Pat. No. 7,861,797, PCT Publication No. WO2012/144500 and German Patent Document No. DE1602006 all disclose such hammer drills. In each of these hammer drills the additional tangential impact mechanism is rotationally driven at a same rate as the rate of rotation of the hammer drills output spindle.
The object of the present invention is to provide a drill with an additional tangential impact mechanism which has an improved operational performance.